The gut microbiota contributes to the protein nutrition of humans and other animals. This study will use the model insect Drosophila melanogaster to determine how hosts interact physiologically and genetically with their gut microbes to maximize protein nutrition as well as overall health and fitness. The basis of this proposal is that the nutritional physiology and symbiosis of animals are interdependent, such that animal protein nutrition can only be understood fully when considered as a symbiotic system. Furthermore, this symbiotic system is highly dynamic: perturbation to the microbiota or diet can have major impact on the protein nutrition, acting through interactions between host nutritional physiology and the composition/function of the microbiota, with profound impacts on the health of the host. Drosophila is particularly amenable to this study because it is highly homologous to humans in its nutritional requirements, but has the experimental advantage of possessing a simple gut microbiota that can be manipulated easily. Drosophila can also be reared on a variety of experimental diets, allowing tests for the effect of diet composition, mimicking dietary behavioral modification in humans. The proposed study will measure the physiological consequences of dietary and symbiotic manipulation on Drosophila on protein nutrition and overall health and fitness. It will test the hypothesis that host genetic variation in the assimilation and allocation of protein precursors underlies variation in dependence on symbiotic associations for maximal health, and will use genomic resources and systems analyses to define the expression networks that underpin symbioses with different impacts on host protein nutrition. This research will achieve the first understanding of the mechanisms underlying natural variation in the significance of the gut symbiosis to host protein nutrition, with direct relevance to understanding human dietary and nutritional conditions and their impacts on health.